1. Field of the Invention
This invention relates to the assembly and test of electro-optic (EO) sensors on a production line, and more particular to a test system, and method of wavefront measurement.
2. Description of the Related Art
Electro-optics (EO) sensors are configured to image scenes from a point source a long distance away, sufficiently far away that the received electro-magnetic wavefront is typically considered to be planar. EO sensors are: typically used in SWIR, MWIR or LWIR bands but may also operate in the visible band. The EO sensor includes an optical telescope, transmissive or reflective, that is designed to focus an incident electro-magnetic wavefront to an image plane. In a typical scene, the scene is composed of planar wavefronts from multiple point sources. The structure of the scene is imprinted on the composite wavefront as a summation of planar wavefronts with different slopes. The telescope converts these slopes to spatial offsets in the image plane to form an image of the scene. An EO detector is mounted to the optical telescope at or near the image plane to convert the image of the scene to an electronic representation. A video card reads out a temporal sequence of frames from the electronic representation to produce a video signal at an output port. The video signal is provided to other systems such as a guidance module that process images of the scene.
Ideally the optical telescope converts the incident wavefront into a spherical wavefront that collapses onto the image plane of the optical system. Given an ideal point source positioned on the optical axis of the telescope, any deviation from the perfect spherical wavefront (i.e. local slope differences of the wavefront) represents a wavefront error that distorts the image in some way and degrades system performance. Typical sources of error include surface shape defects in the optical telescope itself and mechanical stresses on the optical telescope from mounting the EO detector or other components. It is useful to characterize and understand these deviations in order to both qualify EO sensors during production and to mitigate the sources of error (e.g. improved alignment of telescope components, improved mounting of the EO detector to the telescope and so forth). The wavefront measurement may also be used to directly compensate the errors via a deformable mirror in some applications.
During production of an EO sensor or an assembly including an EO sensor, various tests and calibration procedures are performed at each stage of assembly. Prior to any assembly, the bare optical telescope is tested to compute an initial wavefront estimate. This test is typically performed with an interferometer that superimposes a wavefront under test with a reference wavefront. The difference between these wavefronts creates an interference pattern with a series of fringes that can be mapped to the wavefront error.
The test may also be performed with a Shack-Hartman wavefront sensor that illuminates the entire entrance pupil of the telescope with a collimated beam and uses another optic to image the wavefront onto a lenslet array. The lenslet array spatially separates the wavefront into sub-pupils and focuses the sub-pupils simultaneously onto a detector. Each sub-pupil is focused onto a different sub-region on the detector, and the displacement of each sub-region image with respect to an expected position from a desired wavefront can then be related to the local wavefront error. The extent of each sub-region defines the dynamic range for measuring the local wavefront slope; the greater the spatial resolution the smaller the dynamic range. The different sub-regions of the detector are read out in parallel to provide the local wavefront slope measurements across the entire wavefront simultaneously. The measurements are processed to compute a wavefront estimate. These estimation techniques are described by Harrison H. Barrett et al. “Maximum-likelihood methods in wavefront sensing: stochastic models and likelihood functions” Vol. 24, No. 2/February 2007/J. Opt. Soc. Am. 1 pp. 391-414. Shack-Hartman provides greater dynamic range for measuring local wavefront slopes (error) but less spatial resolution than the interferometer.
The EO detector is then typically mounted onto the telescope near the image plane to form the EO sensor. The EO sensor is subjected to a variety of tests and calibration procedures. If a test reveals a problem e.g. a focus test reveals that the EO sensor's modulation transfer function (MTF) does not meet the specification, the unit is pulled off the production line and retested using the interferometer or Shack-Hartman wavefront sensor. In both cases, a collimated beam that fills the entrance pupil is passed through the telescope and reflected off the EO detector back through the telescope to an external detector. In this double-pass configuration, alignment is critical, hence time-consuming and expensive. Both the hardware and operation of the interferometer and Shack-Hartman wavefront sensor are expensive. Both require an external EO detector as part of the hardware package. Both require an experienced engineer to perform the test.